Overvoltage protection and short-circuit withstanding for gallium nitride devices

ABSTRACT

Disclosed examples include methods, integrated circuits and switch circuits including a driver circuit and a silicon transistor or other current source circuit coupled with a gallium nitride or other high electron mobility first transistor, where the driver operatives in a first mode to deliver a control voltage signal to the first transistor, and in a second mode in response to a detected overvoltage condition associated with the first transistor to control the current source circuit to conduct a sink current from the first transistor to affect a control voltage to at least partially turn on the first transistor.

BACKGROUND

The present disclosure relates generally to switch circuits andovervoltage protection for switching devices. High electron mobilitytransistors (HEMTs) are becoming an attractive solution for highefficiency switching power supplies, amplifiers and other switchcircuits due to low on-state resistance (e.g., RDSON). Compared withconventional silicon switching transistors, gallium nitride (GaN),aluminum gallium nitride (AlGaN), silicon carbide (SiC) and other HEMTspromise higher output power, lower size and high efficiency in powercircuits, particularly at high frequencies. However, these technologiesare in early stages of development, and generally have not displacedsilicon solutions. Although well suited for high voltage powerconversion applications, GaN transistors are susceptible to damage ifbiased into breakdown, as present GaN transistors do not have inherentavalanche capability commensurate with many silicon power FETs. Further,some silicon FETs also do not have inherent avalanche capability. Inaddition, many switch circuit applications require the ability towithstand short-circuit conditions, and current GaN fabricationtechniques do not provide adequate short-circuit withstand ability. Onetechnique to incorporate GaN or low inherent avalanche capabilitysilicon transistors is to over design devices with excessive breakdownvoltage margin, so that the transistor is never driven into breakdown.This approach is costly, and may subject the system to excessivevoltages which would otherwise have been clamped with a silicon switchdesign.

SUMMARY

Disclosed example switch circuits include a driver and a current sourcecircuit coupled with a gallium nitride or other transistors, and adriver circuit. In a first mode, the driver circuit delivers a controlvoltage signal to the first transistor. In a second mode, in response toa detected overvoltage condition associated with the first transistor,the driver circuit controls the current source circuit to conduct a sinkcurrent from the first transistor to turn on the first transistor. Thecurrent source facilitates discharging the overvoltage condition in acontrolled manner. In certain examples, the driver operates in responseto a detected overcurrent condition to control the current source toconduct current from the first transistor to limit current flow in thefirst transistor to enable short-circuit withstanding capability of theswitch circuit without permanent damage to the first transistor. Thecurrent source in certain examples is a second transistor coupled withthe first transistor in a cascode configuration. The second transistorin certain examples is fabricated in a silicon die along with the drivercircuit and an overvoltage sense circuit. In other examples, the secondtransistor may also be fabricated monolithically along with the firsttransistor, or the first and second transistors can be monolithicallyintegrated with the driver circuit. In the second mode, the driverprovides a control voltage signal to operate the second transistor in asaturation mode to conduct a sink current from the first transistor andto affect a gate-source control voltage of the first transistor in thecascode configuration to at least partially turn on the firsttransistor. The driver circuit in certain examples implements an analogor digital control of the second transistor in responding to overvoltageconditions, and the second transistor is controlled according to theamount of overvoltage associated with the first transistor. Disclosedexamples can be used in half bridge or other high side/low side powerconverter configurations to facilitate the use of GaN or othertransistor switches for high efficiency systems while providingovervoltage and short-circuit withstand capability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a switch circuit including a galliumnitride high electron mobility transistor and an integrated circuitincluding a driver and a low voltage silicon transistor.

FIG. 2 is a flow diagram of a process for controlling a switch circuitimplemented in the integrated circuit of FIG. 1.

FIG. 3 is an output characteristic diagram illustrating operation of thelow voltage silicon transistor in the integrated circuit of FIG. 1.

FIG. 4 is a simplified partial top plan view of an example low voltagesilicon transistor with an interleaved multiple gate configurationproviding digital control.

FIG. 5 is a schematic diagram of a half bridge power circuit using highand low side gallium nitride transistors and corresponding driverintegrated circuits forming a DC-DC power converter with overvoltage andovercurrent protection.

FIG. 6 is a partial schematic diagram of a circuit for sensingovervoltage conditions in the integrated circuit of FIG. 1.

FIG. 7 is a partial schematic diagram of another example circuit forsensing overvoltage in the integrated circuit of FIG. 1.

FIG. 8 is a waveform diagram illustrating an example overvoltagewaveform and corresponding analog and digital gate control curves forthe low voltage silicon transistor in the switch circuit of FIG. 1.

FIG. 9 is a schematic diagram of an example half bridge switch circuitincluding high and low side gallium nitride transistors and a driverintegrated circuit to use the low side transistor for overvoltage andovercurrent protection.

DETAILED DESCRIPTION

In the drawings, like reference numerals refer to like elementsthroughout, and the various features are not necessarily drawn to scale.In the following discussion and in the claims, the terms “including”,“includes”, “having”, “has”, “with”, or variants thereof are intended tobe inclusive in a manner similar to the term “comprising”, and thusshould be interpreted to mean “including, but not limited to . . . .”Also, the term “couple” or “couples” is intended to include indirect ordirect electrical connection or combinations thereof. For example, if afirst device couples to or is coupled with a second device, thatconnection may be through a direct electrical connection, or through anindirect electrical connection via one or more intervening devices andconnections.

FIG. 1 shows a switch circuit 100 including a first transistor 101coupled in series with a second transistor 102 to form a switch circuitbetween a first circuit node 104 (e.g., a supply voltage node) and asecond circuit node 106 (e.g., ground node GND or the switched node in ahigh-side configuration). The first transistor 101 in one example is agallium nitride transistor (e.g., GaN, AlGaN, etc.). In other examples,the first transistor 101 can be a silicon (Si) silicon carbide (SiC)transistor or other PET. The second transistor 102 in this example is alow voltage silicon transistor that provides a current source toselectively conduct sink current I2 from the first transistor 101 to thesecond circuit node 106 in response to detection of overvoltage and/orovercurrent conditions. In the illustrated example, the current source,including the second transistor 102, is fabricated in an integratedcircuit (IC) 110 along with an overvoltage sense circuit 112 and adriver circuit 116. The driver circuit 116 operates the first and secondtransistors 101 and 102 in multiple modes to provide for normalswitching operation as well as protection against overvoltage and/orovercurrent conditions. In other cases, the second transistor 102 anddriver circuit 110 can be fabricated monolithically on the samesubstrate as that of the first transistor 101, e.g., all transistorsbeing Si, GaN or SiC. In one example, the driver circuit 116 operates ina first mode (e.g., NORMAL mode), a second mode (OVERVOLTAGE) inresponse to detected overvoltage conditions associated with the firsttransistor 101, and a third mode (OVERCURRENT) in response to detectedovercurrent conditions in the switching circuit 100. The overvoltagesense circuit 112 in one example senses a voltage of the first circuitnode 104 and selectively provides an overvoltage detection signal O-V ona line 114 as an input to the driver circuit 116 when the sensed voltageexceeds a threshold level. The overvoltage detection signal O-V causesthe driver circuit 116 to operate in the second mode in response to adetected overvoltage condition. The driver circuit 116 implementsovervoltage/overcurrent protection by selective operation of the secondtransistor 102 to implement a current source in series with the firsttransistor 101 between the first and second circuit nodes 104, 106 asdescribed further below.

The switch circuit 100 in one example can is a dual-die componentincluding a GaN die to form the first transistor 101 and a silicon dieto form the IC 110 including the driver circuit 116, the overvoltagesense circuit 112, and the current source including the secondtransistor 102. The switch circuit 100 can be a single dual-die productwith suitable electrical connections for the first and second circuitnodes 104 and 106, as well as a drive signal input 108 to the drivercircuitry 116 in order to receive a drive signal DRV from an externalcontroller (not shown). In other implementations, the driver IC 110 canbe a separate product for use with a connected HEMT transistor 101. TheIC 110 has a number of pads or pins for electrical interconnection withexternal circuits. The IC 110 in one example includes a first pin or pad131 for electrical coupling (e.g., direct or indirect electricalconnection) with a first source terminal S1 of the first transistor 101,as well as a second pad 132 convey a first gate control voltage signalGC1 from an output 118 of the driver circuit 116 to a first gate controlterminal G1 of the first transistor 101. The IC 110 also includes a pad134 to allow electrical connection to a drain terminal D1 of the firsttransistor 101, as well as electrical connection to the first switchcircuit node 104. The IC 110 in this example includes a pad 135 to allowthe driver circuit 116 to receive the drive signal DRV from an externalcircuit, and the IC 110 includes a positive supply voltage pad 136 forconnection to a supply voltage VDD, and a common connection pad 137 forconnection to a supply reference node COM. In addition, the IC 110includes a pad 138 for connection to the switch circuit ground node 106(GND). In another example, the IC 110 can monolithically include thefirst transistor 101, particularly in a high-voltage silicon, GaN, orSiC IC process, to form a single-die chip.

The switch circuit 100 can be configured in any suitable powerelectronics circuitry, such as half or full bridge switching circuits, amultiphase inverter, etc., to implement a power conversion system. Inone example, as illustrated in FIG. 5 below, a pair of switch circuits100 can be connected in series with one another, with a switching nodejoining the switch circuits 100, in combination with an inductor to forma DC-DC converter circuit or half bridge converter to drive a load. Thedrive signal input 108 for each such switch circuit 100 can be providedwith a pulse width modulated control signal DRV for corresponding highside or low side switching to provide DC-DC conversion. In otherexamples, multiple switch circuits 100 can be interconnected to form asingle or multiphase inverter to drive an AC load by corresponding pulsewidth modulated DRV signals provided to the individual switch circuits100.

The first transistor 101 in FIG. 1 is a GaN field effect transistor(FET) with a first drain terminal D1 connected to the first circuit node104, a first source terminal S1 connected to the second transistor 102via the first pad 131 of the IC 110, and a first control terminal (e.g.,gate) G1 connected to receive the first gate control signal GC1 from thedriver circuit 116 via the IC pad 132. The driver circuit 116 providesthe control signal GC1 in one example according to the DRV signalreceived at the input 108 in the first (e.g., NORMAL) mode forcontrolled switching of the first transistor 101. In certainimplementations, the driver circuit 116 continues to provide the GC1signal according to the DRV signal in the second (e.g., OVERVOLTAGE)mode. In other implementations, the driver circuit 116 provides thesignal GC1 to set the voltage of the first gate terminal G1 to aconstant voltage (e.g., 0 V) in the second mode. In one example, thedriver circuit 116 operates in the second mode in response to a detectedovervoltage condition associated with the first transistor 101 tocontrol the second transistor 102 in a saturation mode to operate as acurrent source to conduct a sink current I2 from the first sourceterminal S1 to the second circuit node 106. The controlled operation ofthe current source 102 sinking current I2 from the first source terminalS1 causes the first transistor 101 to self-adjust the first gate-sourcevoltage Vgs1 to control the current I1 flowing in the first transistor101 to match the current I2 flowing in the current source transistor102. In this manner, the controlled operation of the current sourcetransistor 102 affects the gate-source control voltage Vgs1 to at leastpartially turn on the first transistor 101.

In other examples, the driver circuit 116 provides one or more secondcontrol voltage signals GC2 (e.g., GC2 a and GC2 b in the example ofFIG. 1) along corresponding lines 120 a and 120 b to the secondtransistor 102 according to the DRV signal in the first mode, whileholding the GC1 signal to provide a constant voltage at the gate G1 ofthe first transistor 101 during normal switching operation of the switchcircuit 100. In this regard, the first and second transistors 102 can beoperated as a safety cascode configuration or as a traditional cascodecombination during the first (e.g., NORMAL) operating mode

The second transistor 102 in this example is a low voltage silicon FETtransistor formed in a silicon die of the IC 110 to implement acontrolled current source in series with the first transistor 101. Inthis case, the second transistor 102 includes a second drain terminal D2coupled with the first source terminal S1, a second source terminal S2coupled with the second circuit node 106, and one or more second controlterminals (e.g., gate terminals) G2. The second drain terminal D2 isconnected via a line 124 and the first pad 131 to the source terminal S1of the first transistor 101. The driver circuit 116 is connected to thefloating node 124 connecting the source S1 of the first transistor 101to the drain D2 of the second transistor 102 for detecting short circuitconditions associated with the switch circuit 100.

In the second and third modes, the driver circuit 116 delivers thesecond control voltage signal GC2 to the gate G2 to turn on the secondtransistor 102 to conduct the sink current I2 current from the firstsource terminal S1 to the second circuit node 106 to limit the currentT1 flowing in the first transistor 101. In certain examples, the drivercircuit 116 provides the control voltage signal(s) GC2 according to theamount of overvoltage associated with the first transistor 101. Forexample, the driver circuit 116 can provide a single signal GC2 to asingle gate of the second transistor 102 in the second mode as an analogvoltage signal with an amplitude controlled according to the amount ofovervoltage associated with the first transistor 101. In other examples,multiple second transistors 102 can be connected in parallel with oneanother between the first transistor 101 and the second circuit node106, and the driver circuit 116 provides individual gate control signalsto the gate control terminals of the second transistors 102 in order toimplement digital control to set the sink current I2. In othernon-limiting examples, as described further below in connection withFIG. 2, the second transistor 102 includes multiple gate controlstructures, which can be interleaved with one another in certainimplementations for improved thermal heat-spreading. In such examples,the driver circuit 116 provides separate control signals (e.g., GC2 aand GC2 b in FIG. 1; CG2 a, GC2 b and GC2 c in FIG. 4) to implementdigital control of the sink current I2 according to the amount ofovervoltage associated with the first transistor 101. In certainembodiments, the drive circuit 116 operates the second transistor(s) 102in a linear mode during the first operating mode to set the on-stateimpedance of the second transistor 102 (e.g., RDSON) to a low value fornormal operation, and operates the second transistor or transistors 102in a saturation mode to implement a current source in the second andthird operating modes to address overvoltage and/or overcurrentconditions in the switch circuit 100.

Referring now to FIGS. 2 and 3, FIG. 2 shows a process or method 200 foroperating the switch circuit which can be implemented in the IC 110 inone example, and FIG. 3 illustrates a graph 300 including curves 301-306showing linear and saturation region operation of the second transistor102. In one possible implementation, the driver andovervoltage/overcurrent protection circuit 116 in FIG. 1 includessuitable logic, programmable or otherwise, to implement the process 200of FIG. 2.

Normal mode operation is implemented in the first mode at 202, in whichthe driver circuit 116 delivers the signal GC1 to control the firsttransistor (e.g., GaN) according to the DRV signal. In this example, thedriver circuit 116 provides the second control signal or signals GC2 toset Vgs2 to a relatively high first predetermined value. In oneembodiment, the predetermined value is set to drive the secondtransistor 102 (Si FET) in a linear region to provide a minimal orrelatively low RDSON to provide a low impedance between the firsttransistor 101 and the second circuit node 106. This minimizes the powerconsumption of the second transistor 102 in normal operation, therebyfacilitating efficient operation of the switch circuit 100. In thisexample, the first transistor 101 operates as the circuit switchaccording to the DRV signal, and most of the switch circuit voltage dropbetween the circuit nodes 104 and 106 is across the first transistor 101when the switch 101 is off. In certain examples, therefore, the secondtransistor 102 can be a low voltage silicon transistor, and the firsttransistor 101 is designed to accommodate the expected operatingvoltages and currents of a power conversion system or other host systemin which the switch circuit 101 is employed.

In another example at 202 in the first (NORMAL) mode, the driver circuit116 provides the control signal GC1 to ground the first transistor gateG1 (or to set the voltage of the gate G1 to another constant voltagesuch as with reference to its source or the switching node 131 in FIG.1), and the circuit 116 provides the control signal or signals GC2 inorder to control the second transistor 102 according to the DRV signal.

A determination is made at 204 as to whether an overvoltage conditionhas been sensed or detected. In one example, the overvoltage sensecircuit 112 measures or senses the voltage at the first circuit node 104(e.g., the drain D1 of the first transistor 101), and compares thevoltage at the node 104 with a threshold voltage (not shown) at 204 inFIG. 2. For example, the overvoltage sense circuit 112 in one exampledetermines whether the voltage across the first transistor 101 isgreater than the threshold value. If no overvoltage condition exists (NOat 204), the driver circuit 116 determines at 208 whether an overcurrentcondition has been detected. If no overvoltage or overcurrent conditionsare detected (NO at 204 and 208), the driver circuit 116 continuesnormal operation at 202 as previously described.

If an overvoltage condition associated with the first transistor 101 isdetected (YES at 204), the driver circuit 116 operates in the secondmode at 206. In one example, in response to the overvoltage detectionsignal O-V from the sense circuit 112, the driver circuit 116 controlsthe current source by providing the signal(s) GC2 at 206 to cause thetransistor 102 to conduct a non-zero sink current I2 from the firsttransistor 101 to the second circuit node 106. This controlled sinkcurrent I2 affects the voltage Vgs1 and causes the first transistor 101to at least partially turn on. In one example at 206, the driver circuit116 controls the transistor current source 102 according to the amountof overvoltage associated with the first transistor 101 using analog ordigital control. In one example, the driver circuit 116 sets the gatevoltage to zero or another constant value via the signal GC1 to controlthe first transistor 101 at 206. In other examples, the driver circuit116 continues to deliver the gate control signal GC1 according to theDRV signal in the second mode.

The graph 300 in FIG. 3 shows curves 301-306 corresponding to differentgate-source voltages Vgs2 in linear and saturation modes or regions foroperation of the second transistor 102. In certain examples, the drivercircuit 116 provides one or more gate control signals GC2 to control thegate-source voltage Vgs2 of the transistor 102 in the linear region inthe first mode at 202 at a first predetermined value set according to adesired low on state resistance (e.g., low RDSON). For example, thedriver circuit 116 in the first normal mode operation may provide agate-source corresponding to the curve 306 at a relatively lowdrain-source voltage VD2S2 having a fairly high slope in the linearregion of FIG. 3, while the first transistor 101 (FIG. 1) is operatedaccording to the DRV signal. Curves 305, 304, 303, 302 and 301correspond to successively lower gate-source voltages Vgs2 of thetransistor 102. In the second mode, in response to detected overvoltageconditions, the driver circuit 116 the in one example lowers the secondgate control signal or signals GC2 in order to operate the secondtransistor 102 at a lower gate-source voltage Vgs2, for example, in thesaturation region along the curve 303. Operation in the second mode inthe saturation region advantageously controls the second transistor 102as a current source to sink current I2 out of the source S1 of the firsttransistor 101. This facilitates discharge of the overvoltage condition,with the sink current I2 affecting the first transistor gate-sourcevoltage Vgs1, causing the first transistor 101 to self-adjust itsoperation to conduct the same amount of current (e.g., I1 isapproximately equal to I2). In this manner, the driver circuit 116controls the amount of overvoltage discharge current by setting the gatevoltage of the second transistor 102 to a lower second predeterminedvoltage corresponding to the desired on-current I2 in the saturationregion.

Returning to the process 200 in FIG. 2, the driver circuit 116 continuesto monitor the overvoltage detection signal O-V from the overvoltagesense circuit 112 at 204, and continues operation in the second modewhile the overvoltage condition remains. If the overvoltage condition iscleared (NO at 204), the overvoltage sense circuit 112 discontinues thesignal O-V, and the driver circuit 116 determines at 208 whether anovercurrent condition is detected. In one example, the driver circuit116 compares the voltages at the nodes 104 and 124 to determine whethera short circuit condition has been detected, indicating an overcurrentcondition in the switch circuit 100. In another example, the voltagebetween nodes 124 and 138 is compared. This determination in one examplecorresponds to determining whether the current I1 exceeds a thresholdcurrent ITH shown in the graph 300 of FIG. 3. If not (NO at 208), theprocess 200 returns to normal operation in the first mode at 202 aspreviously described.

If an overcurrent condition has been detected (YES at 208), the drivercircuit 116 begins operation in a third (e.g., OVERCURRENT) mode. In oneexample, the driver circuit 116 sets the gate voltage of the firsttransistor 101 at 210 to zero or another constant voltage. In otherexamples, the driver circuit 116 continues to control the gate voltageof the first transistor 101 according to the DRV signal at 210. In thethird mode, the driver circuit 116 controls the second transistorgate-source voltage Vgs2 to a low third predetermined voltage foroperating the transistor 102 in the saturation mode or region. Theselected gate-source voltage corresponds to a desired current limit, forexample, along the curve 305 slightly above the threshold level ITH inone implementation. In certain implementations, the driver circuit 116maintains operation at 210 for a predetermined time (e.g., 10 μs), andthen shuts down the switch circuit by turning off both transistors 101and 102 at 212. In other implementations, the driver circuit 116continues to monitor the presence or absence of an overcurrent conditionat 208.

If the overcurrent condition is cleared (NO at 208), the driver circuit116 returns for normal operation at 202 as described above. In thismanner, the driver circuit 116 detects switch circuit current exceedingthe threshold ITH when there is no overvoltage condition, correspondingto detection of a short circuit condition. In response to the detectedovercurrent condition, the driver circuit 116 clamps the switch circuit100 at a predetermined maximum current by operating the secondtransistor 102 in the saturation region to operate the switch circuit ina cascode configuration, and the driver circuit 116 can maintain thismaximum current operation for a certain time to allow the possibility ofthe overcurrent condition clearing, or the driver circuit 116 may shutdown the switch circuit 100 after a predetermined time. The IC 110 thusimplements overcurrent protection for switch circuits 100 using GaN orother HEMT switching transistors 101 to meet any desired short circuitwithstand rating, and also provides avalanche operation to protect thetransistor 101 in overvoltage conditions, facilitating the use ofswitching transistors 101 that do not inherently provide avalanchebehavior and short circuit withstand capability suitable to a givenapplication.

Referring also to FIG. 4, in one example the second transistor 102 is asilicon FET with a multiple gate construction allowing digital controlby the driver circuit 116. FIG. 4 shows a simplified top view in which adrain structure D2 and a source structure S2 are formed in a silicon dieor wafer, and first through third conductive gate structures 120 a, 120b and 120 c form an interleaved gate structure G2 overlying channelregions between fingers of the drain D2 and the source S2. In thisexample, the driver circuit 116 provides first, second and third gatecontrol signals GC2 a, GC2 b and GC2 c to the gate structures 120 a, 120b and 120 c, respectively, and the driver circuit 116 separatelycontrols the voltage applied to the gate structures 120. The drivercircuit 116 in one example separately implements analog control of thevoltage applied to each of the gate structures 120 a-120 c, and canprovide the gate control signals GC2 a, GC2 b and GC2 c at individualvoltages based at least in part on the amount of overvoltage detected inthe second operating mode. In another example, the driver circuit 116provides the gate control signals GC2 a, GC2 b and GC\2 c atpredetermined voltages, and separately drives none, one, or both of thegate structures 120 a and 120 b to implement digital control, with thedigital or binary actuation of the multiple gate structure 120implemented to control the operation of the second transistor 102 atleast partially according to the amount of detected overvoltageassociated with the first transistor 101 in the second mode. The drivercircuit 116 can implement digital control to selectively turn on or shutoff portions of the second transistor 102 in order to control thesaturation current of the second transistor 102 for a given gatevoltage. For example, the driver circuit 116 turns on one of the gateportions 120 a or 120 b to control operation of the switch circuit 100to implement a controlled current source at a first current level I1/I2,and can turn on the other gate portion 122 implement a second highercurrent level I1/I2. The example of FIG. 4 and other interleavedembodiments advantageously interleave the gate structure portions 120 aand 120 b to reduce self-heating of the low voltage silicon transistor102. The following Table 1 shows a digital control scheme example usingthree FET portions of different sizes, for example, as shown in FIG. 4.

TABLE 1 segment fill % code 120a (GC2a) 10 0 1 0 1 0 1 0 1 120b (GC2b)30 0 0 1 1 0 0 1 1 120c (GC2c) 60 0 0 0 0 1 1 1 1 % of 2ND FET 0 10 3040 60 70 90 100 102 on

FIG. 5 shows another embodiment including a half bridge power conversionsystem or circuit 500 including first and second (e.g., high and lowside) GaN transistors 101 a and 101 b and corresponding first and seconddriver ICs 110 a and 110 b in switch circuits 100 a and 100 b,respectively. The integrated circuits 110 a and 110 b individuallyinclude an overvoltage sense circuit 112, a driver andovervoltage/overcurrent protection circuit 116, and a low voltagesilicon second transistor 102 as described above in connection withFIG. 1. An input voltage VIN is connected to the first circuit node 104a of the first switch circuit 100 a, and this input voltage is providedto the terminal 134 of the first IC 110 a and the drain D1 of the firsttransistor 101 a of the first switch circuit 100 a. The first transistor101 a of the first switch circuit 100 a conducts a drain-source currentI1 a, and the internal second transistor of the IC 110 a operatesaccording to control signals from the corresponding driver circuit 116in the first, second and third modes as described above. The firstdriver IC 110 a includes a terminal 135 connected to receive a firstdrive control signal DRVa along a line 108 a from a power conversioncontrol circuit 502, and selectively operates the first transistor 101 aor the internal second transistor 102 according to the drive signal DRVain the first (e.g., NORMAL) mode as described above.

A second circuit node 106 a associated with the first switch circuit 100a is connected from a pad 138 of the first driver IC 110 a to a firstcircuit node 104 b of the second switch circuit 100 b to form a switchnode in the power conversion circuit 500. An output inductor L isconnected between the switch node 106 a, 104 b and a load 504. In normaloperation, absent any overvoltage or overcurrent conditions, the firstand second switch circuits 100 a and 100 b are operated respectively ashigh and low side switches to provide a DC output voltage VOUT acrossthe load 504 with respect to a power converter ground node GND.

The second switch circuit 100 b operates as a low side switch, andincludes a driver IC 110 b, and a first low side transistor 101 b with adrain D1 connected to the switch node 104 b and a source S1 connected toa pad 131 of the second driver IC 110 b. The transistor 101 b conducts adrain-source current I1 b as shown, and the second driver IC 110 bincludes a driver circuit 116 and an internal second transistor 102 asdescribed above in connection with FIG. 1. The source S2 of the internalsecond transistor 102 of the second driver IC 110 b is connected to theground node GND through a corresponding pad 138 and second circuit node106 b as shown in FIG. 5. The second switch circuit 100 b receives asecond drive signal DRVb along a line 108 b from the control circuit502.

In operation in one example, the control circuit 502 generates the drivesignals DRVa and DRVb to provide alternate connection of the switch node106 a, 104 b to the input voltage VIN or to the ground GND, with therelative on-times of the drive signals DRVa and DRVb regulating theoutput voltage VOUT provided to the load. The control circuit 502receives a feedback signal representing the output voltage VOUT andselectively adjusts the pulse width modulation (e.g., on-times) of thedrive signals DRVa and DRVb in a closed loop fashion to regulate theoutput voltage VOUT.

Referring now to FIGS. 1, 5 and 6, the illustrated driver ICs 110include an internal overvoltage sense or detection circuit 112. FIG. 6shows an example overvoltage sense circuit 112 to sense overvoltageconditions associated with a connected first transistor 101. In thisexample, the avalanche capability of a silicon P-N junction formedbetween N and P regions of a silicon die used in forming the integratedcircuit 110 is used along with a resistor circuit R1, R2 to detectovervoltage conditions associated with the switch circuit 100 and thefirst transistor 101 thereof. The first resistor R1 is connected to thefirst circuit node 104 at the drain (D1) of the first transistor 101(e.g., FIG. 1), and the second resistor R2 is connected between R1 andthe N-type doped region 602. The P-type doped region 600 can be formedusing any suitable semiconductor fabrication techniques in asemiconductor body or wafer, such as implantation of P-type dopants in asilicon wafer. The N-type doped region 602 can be formed as an implantedwell or other N-type doped region 602 having at least one boundary orjunction with the P-type doped region 600 forming a P-N junction.

In operation, when the voltage VD exceeds a certain avalanche thresholdassociated with the silicon P-N junction 600/602, the avalanche behaviorof the silicon die causes conduction through reverse biasing of thecorresponding diode of the junction. Once this avalanche current flowreaches a certain threshold, set by the resistive divider circuit valuesof R1 and R2, the voltage at the node joining R1 and R2 will rise abovea threshold level. In one example, the overvoltage sense circuitry 112includes a level shift circuit 604, although this circuit 604 can beomitted in certain embodiments. For example, if the integrated circuit110 includes a high voltage junction with low side substrate currentI_sub access, the level shift circuit 604 is not needed. The circuit 112in this example also includes a comparator circuit 606 with a firstinput (−) to receive a sense signal OVM representing the magnitude oramount of overvoltage directly from the node joining R1 and R2 or froman intervening level shift circuit 604 representing a voltage of a nodejoining the first and second resistors R1 and R2. A second comparatorinput (+) is connected to receive a threshold voltage signal VTH. Theoutput of the comparator 606 delivers the overvoltage detection signalO-V to cause the driver circuit 116 to operate in the second mode whenthe sense signal exceeds the threshold voltage signal VTH. The circuit116 in certain examples uses analog or digital control according to theovervoltage magnitude signal OVM to implement proportional analog ordigital control.

FIG. 7 shows another example overvoltage sense circuit 112 for sensingovervoltage in the integrated circuits 110 of FIGS. 1 and 5. Thisexample includes a varistor 700 (e.g., a metal oxide varistor or MOV)the connected between a lower level supply voltage (e.g., VDD in FIG. 1above) and a node 702. In other examples, the varistor 700 can bereplaced by a Zener diode (not shown). In this example, the overvoltagesense circuit 112 does not require a high voltage connection, butinstead can be connected to a supply level of the IC 110 (e.g., VDD). Inthis regard, lightning or other source of overvoltage spikes will bereflected on the VDD supply voltage, and the circuit 112 uses this spiketo detect overvoltage conditions associated with the first transistor101 of the connected transistor 101 in the associated switch circuit100. As seen in FIG. 7, a resistor R3 is connected between the node 702and the associated common connection COM, and excessive voltages on theVDD node will cause conduction through the varistor or Zener diode 700.Such current flows through the resistor R3, and causes a voltage rise onthe node 702. The circuit 112 in this example further includes acomparator circuit 606 with a first input (−) to receive a sense orovervoltage magnitude signal OVM representing the voltage of the node702, a second input (+) to receive a threshold voltage signal VTH, andan output to deliver the overvoltage detection signal O-V to cause thedriver circuit 116 to operate in the second mode OVERVOLTAGE when thesense signal exceeds the threshold voltage signal VTH.

FIG. 8 shows a waveform or graph 800 illustrating an example overvoltagewaveform 802 and corresponding analog and digital low voltage transistorgate control curves 804 and 806 to control the second transistor 102 inthe switch circuit 100 of FIG. 1. In this example, the drain voltage VDof the first transistor 101 is brought to a peak or crest shown as ascaled value 1.0, and the curve 802 decreases over time for acombination wave generator CW G1.2/50 voltage surge test waveformaccording to an ANSI IEEE C62.41-1991 transient overvoltage immunitytest. The driver circuit 116 responds to detected overvoltage conditionsassociated with the first transistor 101 to control the secondtransistor 102 using analog or digital control in certain examples asdescribed above. The curve 804 in FIG. 8 shows analog control of thesecond gate source voltage Vgs2 using the control signal GC2 in FIG. 1,where the driver circuit 116 adjusts the amplitude of the applied gatecontrol signal GC2 over time at least partially according to the amountof overvoltage shown in the curve 802. A stepped curve 806 in FIG. 8illustrates a 2-bit digital control implementation via the controlsignals GC2 a and GC2 b in FIG. 1, which also varies over time accordingto the overvoltage amount in the curve 802. Multi-value or digitalcontrol advantageously reduces ringing since the overvoltage mitigationis less sudden. In addition, digital control provides better ability towithstand process variability of the threshold voltage of the secondtransistor 102.

FIG. 9 shows an example half bridge switch circuit 900 including highand low side gallium nitride transistors 901 and 902, as well as adriver IC 910 controlling the transistors 901 and 902. In this example,the transistors 901 and 902 are both GaN FETs or other HEMTs coupled inseries with one another between an input voltage VIN at a first circuitnode 104 a and a power converter ground node GND at a second circuitnode 106 b. The low side transistor 902 in this example has aninterleaved, two element control gate. A center node joins thetransistors 901 and 902 to form a switching node connected to drive aload 904 through an output inductor L. The IC 910 receives first andsecond drive control signals DRVa and DRVb along lines 908 a and 908 bvia IC pads 935 a and 936 b from a power conversion circuit controller906. The IC 910 selectively operates the transistors 901 and 902according to the drive signals DRVa and DRVb in a normal or first modeas previously described. In addition, the control circuit 906 in oneexample provides the drive signals DRVa and DRVb in alternating fashionto implement high side and low side operation of the first and secondtransistors 901 and 902 in order to regulate the output voltage VOUTprovided to the load 904 in closed loop fashion. The IC 910 includes apad 935 for connection to a supply voltage VDD, as well as a commonconnection pad 937. In addition, a pad 932 delivers a gate controlsignal to the gate G1 of the first transistor 901, first and second pads933 a and 933 b provide one or more gate control signals to a gate G2 ofthe second transistor 902, and a pad 931 is connected to a switchingnode joining the source S1 of the first transistor 901, the drain D2 ofthe second transistor 902, and the output inductor L.

The IC 910 also includes overvoltage sense circuitry, such as thecircuit 112 described above, to selectively detect overvoltageconditions associated with the switch circuit 900. In one example, theIC 910 includes a pad 934 connected to the drain D1 of the firsttransistor 901 to sense the voltage of the first transistor 901 andselectively detects whether an overvoltage condition exists. In responseto detected overvoltage is associated with the switch circuit 900, theIC 910 controls the low side transistor 902 to operate as a currentsource or current clamp by turning on one or both of a pair of gatecontrol terminals by providing corresponding gate control signals tooutput pads 933 a and 933 b. In operation, this causes the secondtransistor 902 to operate as a controlled current source in a secondmode (OVERVOLTAGE) and/or in a third mode (OVERCURRENT) in response to adetected or sensed overvoltage or overcurrent condition in the switchcircuit 900. In this manner, the IC 910 operates generally as describedabove in connection with FIG. 2 to provide multimode operation, withovervoltage and/or overcurrent protection provided by operation of thesecond transistor 902 as a controlled current source in a cascodeconfiguration with the upper or first transistor 901.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. Modifications are possible in the describedembodiments, and other embodiments are possible, within the scope of theclaims.

The following is claimed:
 1. A circuit, comprising: a first transistor,including a first drain terminal coupled with a first circuit node, afirst source terminal, and a first control terminal; a current sourcecircuit coupled between the first source terminal and a second circuitnode; and a driver circuit operative in a first mode to deliver a firstcontrol voltage signal to the first control terminal, and in a secondmode in response to a detected overvoltage condition associated with thefirst transistor to control the current source circuit to conduct a sinkcurrent from the first source terminal to the second circuit node toaffect a control voltage between the first source terminal and the firstcontrol terminal to at least partially turn on the first transistor. 2.The circuit of claim 1, wherein the first transistor is a galliumnitride (GaN) field effect transistor (NET).
 3. The circuit of claim 1,wherein the current source circuit includes a second transistor, thesecond transistor including: a second drain terminal coupled with thefirst source terminal, a second source terminal coupled with the secondcircuit node, and a second control terminal; and wherein the drivercircuit is operative in the second mode to deliver a second controlvoltage signal to the second control terminal to turn on the secondtransistor to conduct the sink current from the first source terminal tothe second circuit node to limit a first current flowing in the firsttransistor.
 4. The circuit of claim 3, wherein the driver circuit isoperative in the second mode to provide the second control voltagesignal to the second control terminal according to an amount ofovervoltage associated with the first transistor.
 5. The circuit ofclaim 3, wherein the second transistor includes multiple second controlterminals; and wherein the driver circuit is operative in the secondmode to provide multiple second control voltage signals to the secondcontrol terminals using digital control to turn on the second transistoraccording to an amount of overvoltage associated with the firsttransistor to turn on one or more subsections of the second transistor.6. The circuit of claim 3, wherein the driver circuit and the secondtransistor are fabricated in a silicon die, and wherein the firsttransistor is fabricated in a second die using gallium nitride (GaN). 7.The circuit of claim 6, wherein the silicon die includes an overvoltagesense circuit to deliver an overvoltage detection signal to cause thedriver circuit to operate in the second mode in response to detection ofthe overvoltage condition associated with the first transistor.
 8. Thecircuit of claim 7, wherein the overvoltage sense circuit includes: aP-type doped region of a silicon substrate of the silicon die; an N-typedoped region at least partially surrounded by the P-type doped region ofthe substrate of the silicon die; first and second resistors connectedin series with one another between the first circuit node and the N-typedoped region; a comparator circuit including a first input to receive asense signal representing a voltage of a node joining the first andsecond resistors, a second input to receive a threshold voltage signal,and an output to deliver the overvoltage detection signal to cause thedriver circuit to operate in the second mode when the sense signalexceeds the threshold voltage signal.
 9. The circuit of claim 7, whereinthe overvoltage sense circuit includes: a varistor or Zener diodecoupled between a first voltage node of the silicon die and anintermediate node; a resistor coupled between the intermediate node anda second voltage node of the silicon die; and a comparator circuitincluding a first input to receive a sense signal representing a voltageof the intermediate node, a second input to receive a threshold voltagesignal, and an output to deliver the overvoltage detection signal tocause the driver circuit to operate in the second mode when the sensesignal exceeds the threshold voltage signal.
 10. The circuit of claim 7,wherein the overvoltage sense circuit, the driver circuit, the secondtransistor and the first transistor are fabricated monolithically. 11.The circuit of claim 3, wherein the driver circuit is operative in thesecond mode to deliver the second control voltage signal to the secondcontrol terminal to operate the second transistor in a saturation mode.12. The circuit of claim 3, wherein the driver circuit is operative in athird mode in response to a detected overcurrent condition associatedwith the first transistor to deliver a second control voltage signal tothe second control terminal to operate the second transistor in asaturation mode to conduct the sink current from the first sourceterminal to the second circuit node to limit a first current flowing inthe first transistor.
 13. The circuit of claim 1, wherein the drivercircuit is operative in a third mode in response to a detectedovercurrent condition associated with the first transistor to controlthe current source circuit to conduct a sink current from the firstsource terminal to the second circuit node to limit a first currentflowing in the first transistor.
 14. A method of operating a switchcircuit, the switch circuit including a first transistor connected to afirst circuit node, and a current source connected between the firsttransistor and a second circuit node, the method comprising: in a firstmode, delivering a first control voltage signal to the first transistoraccording to a drive signal; in the first mode, controlling the currentsource to provide a low impedance between the first transistor and thesecond circuit node; in a second mode, in response to detection of anovervoltage condition associated with the first transistor, controllingthe current source to conduct a sink current from the first transistorto the second circuit node to at least partially turn on the firsttransistor.
 15. The method of claim 14, further comprising, in thesecond mode, controlling the current source according to an amount ofovervoltage associated with the first transistor.
 16. The method ofclaim 14, further comprising, in the second mode, controlling thecurrent source digitally to turn on one or more subsections of thesecond transistor.
 17. The method of claim 14, further comprising, in athird mode, in response to detection of an overcurrent conditionassociated with the first transistor, controlling the current source toconduct the sink current from the first transistor to the second circuitnode to limit a first current flowing in the first transistor.
 18. Anintegrated circuit to control a switch circuit, the integrated circuitcomprising: a first pad to electrically couple with a first sourceterminal of a first transistor of the switch circuit; a second pad todeliver a first control voltage signal to a first control terminal ofthe first transistor; an overvoltage sense circuit to deliver anovervoltage detection signal in response to detection of an overvoltagecondition associated with the first transistor; and a driver circuitoperative in a first mode to deliver the first control voltage signal tothe second pad to control the first transistor, and in a second mode inresponse to the overvoltage detection signal to control a current sourcecircuit to conduct a sink current from the first source terminal toaffect a control voltage between the first pad and the second pad to atleast partially turn on the first transistor.
 19. The integrated circuitof claim 18, wherein the current source circuit includes a secondtransistor formed in the integrated circuit, the second transistorincluding a second drain terminal coupled with the first pad, and asecond control terminal; and wherein the driver circuit is operative inthe second mode to deliver a second control voltage signal to the secondcontrol terminal to turn on the second transistor to conduct the sinkcurrent from the first source terminal to limit a first current flowingin the first transistor.
 20. The integrated circuit of claim 18, whereinthe overvoltage sense circuit includes: a P-type doped region of asilicon substrate of the integrated circuit; an N-type doped region atleast partially surrounded by the P-type doped region of the substrateof the integrated circuit; first and second resistors connected inseries with one another between the second pad and the N-type dopedregion; a comparator circuit including a first input to receive a sensesignal representing a voltage of a node joining the first and secondresistors, a second input to receive a threshold voltage signal, and anoutput to deliver the overvoltage detection signal to cause the drivercircuit to operate in the second mode when the sense signal exceeds thethreshold voltage signal.
 21. The integrated circuit of claim 18,wherein the overvoltage sense circuit includes: a varistor or Zenerdiode coupled between a first voltage node of the integrated circuit andan intermediate node; a resistor coupled between the intermediate nodeand a second voltage node of the integrated circuit; and a comparatorcircuit including a first input to receive a sense signal representing avoltage of the intermediate node, a second input to receive a thresholdvoltage signal, and an output to deliver the overvoltage detectionsignal to cause the driver circuit to operate in the second mode whenthe sense signal exceeds the threshold voltage signal.